A new study from Tel Aviv University has revealed that sponges in the Gulf of Eilat have developed a unique method to ward off predators. The researchers found that the sponges contain an unprecedented concentration of molybdenum (Mo), a highly toxic mineral. They also identified the bacteria that allows the sponges to store such high concentrations of the precious metal and unveiled the symbiosis between the two organisms.
The study was conducted by PhD student Shani Shoham and Professor Micha Ilan from the School of Zoology at Tau University. The paper was published in the journal Scientific progress.
The researchers explain that sponges are the first multicellular organisms known to science. They live in marine environments and play an important role in the Earth’s carbon, nitrogen and silicon cycles.
A sponge can process and filter seawater worth 50,000 times its body weight every day. With such large amounts of water flowing through it, it can accumulate various trace elements. Scientists are trying to understand how it copes with toxic amounts of substances such as arsenic and molybdenum.
Shani Shoham, PhD student, explains: “20 to 30 years ago, researchers in our lab collected samples of a rare sponge called Theonella conica from the Zanzibar coral reef in the Indian Ocean and found a high concentration of molybdenum. Molybdenum is a trace element, important for the metabolism of cells in all animals, including humans, and widely used in industry.
“As part of my research, I wanted to check whether such high concentrations were also found in this species of sponge from the Gulf of Eilat, where it grows at a depth of more than 27 meters. When I found the sponge and analyzed its composition, I discovered that it contained more molybdenum than any other organism on earth: 46,793 micrograms per gram of dry weight.”
Shoham adds: “Like all trace elements, molybdenum is toxic when its concentration is higher than its solubility in water. But we must not forget that a sponge is essentially a hollow mass of cells without organs or tissues. More precisely, in Theonella conica, up to 40% of the body volume is made up of a microbial society: bacteria, viruses and fungi live in symbiosis with the sponge.”
“One of the most dominant bacteria, called Entotheonella sp., serves as a ‘detoxification organ’ to accumulate metals in the bodies of its sponge hosts. As the bacteria accumulate more and more molybdenum, they convert it from its toxic soluble state to a mineral. We don’t know exactly why they do this. Perhaps the molybdenum protects the sponge by announcing, ‘I’m toxic! Don’t eat me!’ and in return for this service, the sponge doesn’t eat the bacteria and instead serves as their host.”
Molybdenum is in high demand, especially for alloys (such as making high-strength steel), but according to Shoham, it would be impossible to extract it from sponges. Shoham explains: “The concentration is very high, but if you translate it into weight, you can only extract a few grams from each sponge, and the sponge itself is relatively rare. Sponges are cultivated in marine agriculture, mainly for the pharmaceutical industry, but it is quite a difficult undertaking. Sponges are very delicate creatures that need specific conditions.”
“On the other hand, future research should focus on the ability of Entotheonella sp. bacteria to accumulate toxic metals. A few years ago, our laboratory discovered huge concentrations of other toxic metals, arsenic (As) and barium (Ba), in a close relative of Theonella conica, called Theonella swinhoei, which is common in the Gulf of Eilat.
“In this case too, it was discovered that Entotheonella was largely responsible for retaining metals and transforming them into minerals, thus neutralizing their toxicity. Further research on this bacterium could prove useful in treating water sources polluted by arsenic, a serious hazard that directly affects the health of 200 million people worldwide.”
More information:
Shani Shoham et al., Out of the blue: Hyperaccumulation of molybdenum in the Indo-Pacific sponge Theonella conica, Scientific progress (2024). DOI: 10.1126/sciadv.adn3923
Provided by Tel Aviv University
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